Heretofore, in digital transmission systems, so-called high rate error-correcting codes were employed in an attempt at correcting errors in transmission. For example, a binary t error-correcting code can correct t binary errors located anywhere within the code block. If t.gtoreq.3, the code could correct all three bits in an eight-level information bit symbol. For a Gray-coded system that is operating at a low error rate, this capability is not needed, because almost all errors are adjacent symbol errors. That is, a symbol is received as one that is adjacent to the transmitted symbol, thereby producing a single bit error. As is known, a high rate error-correcting code becomes useless at very high received error rates when nonadjacent information bit symbol errors do occur. Thus, the capability of such a code to correct multibit errors within an information bit symbol may never be used. Indeed, error-correcting encoders and decoders employed in such prior arrangements are complex and inefficient.